Expansion and peripheral devices such as flash-memory and radio transceivers are often connected to a host such as a personal computer (PC) by an expansion or peripheral bus such as Universal-Serial-Bus (USB). Such peripherals may plug directly into a USB socket on the host, or may have a cable with a plug that fits into the host's USB connector socket.
Newer versions of peripheral standards such as USB 3.0 offer higher speed transfers through faster clock rates and more efficient framing and handshaking. Burst transfers may further improve throughput. Newer USB devices provide faster transfers when inserted into a host. The host may be able to detect USB devices and reduce throughput for these older devices when inserted into the more advanced host.
Some peripherals are very power sensitive. The peripheral may not have a power supply, or may use a battery. For these low-power peripherals, power consumption may be more important than speed. This may especially be true for flash-memory devices that are already limited in transfer speeds by the relatively slow access of the flash memory.
Some USB devices are embedded within the host system. The capacitance of the long USB cable is eliminated, replaced by a short cable, connector, or metal printed-circuit board (PCB) traces within the host, or even without a cable for directly plugging in. Since the USB standard is designed to drive a relatively long USB cable, when the cable is absent power is wasted when large-current drivers are still used to drive the short embedded cable, connector, or PCB traces.
Since memory on a USB device may be busy or slow, sometimes the host's request cannot be processed immediately. The host may send the request, then periodically poll the USB device to see whether the data is ready. Also, when the host is idle, the host may need to periodically poll the USB device to see if the USB device needs to transfer information to the host. This periodic polling may be used for other purposes as well, such as for polling a mouse for movement.
While polling is useful, since it allows the host to completely control the USB bus, power is consumed each time a packet is sent for polling. While this power is small, for low-power or battery-powered devices, the amount of power consumed may be significant and undesirable. Also, the USB device or host may otherwise be in a low-power sleep or suspend state, and have to wake up into a higher-power state to perform or respond to the polling. There may be significant time and energy required to wake up from the suspend or sleep state, and then to re-enter the suspend or sleep state once polling is done.
What is desired is a low-power peripheral that can be accessed by a high-speed host. It is desirable to reduce power consumption for low-power USB devices that are accessed by a USB host. It is desired to provide a low-power USB connection within the host enclosure to an embedded USB device.